X-ray inspection systems provide precise and fast analysis of solder integrity on loaded printed circuit boards. These X-ray systems typically have a fixed X-ray source and detection means. The printed circuit board (PC board) to be tested is typically mounted in an XY stage, such that the PC board can be moved in the X and Y direction underneath the fixed X-ray source. This allows the PC board to be completely scanned, by the X-ray source, over the PC board's area. Typically the XY stages are motor driven so that the scanning of the PC board can be done under the control of a computer.
A typical XY stage consists of a base and two movable platforms, where the movement of the two platforms is restrained to be in orthogonal directions. There are a number of known methods to restrain the platform's movement. Typically a rod and bearing mounting system is used. Other methods are available. For example, a platform could be mounted on wires or belts that move around pulleys. Or there could be grooves or tracks that a platform slides along. The platforms also typically have a means to move the platforms along their respective orthogonal directions. Some examples are a drive screw, a belt and pulley system, or a gear and track system. Some XY systems combine the orthogonal restricting means and the movement or drive means. For example, an X or Y stage that is suspended from and moved by a cable or belt routed around a number of pulleys combines the moving means and the mechanism that restricts the motion of the X or Y stage.
A conventional XY stage is shown in FIG. 1 and generally consists of a base 102, an Y stage 124, and an X stage 104. The Y stage 124 is mounted on a first pair of rods 114 and is driven along the rods 114 by a first belt 120 attached to the Y stage by clamp 126. The belt 120 is driven by a first motor 116 mounted to the X stage 104.
The X stage 104 is mounted on a second pair of rods 106 that are substantially perpendicular to rods 114. The X stage is driven along the rods 106 by a second belt 110, attached to the X stage by a clamp 112. A second motor 108, mounted to the base 102, drives belt 110. Because the first motor 116 that drives the Y stage 124 is mounted on the X stage 104, the second motor 108 must move the mass of the first motor when it moves the X stage 104. This weight or mass of the first motor 116 increases the minimum size of the second motor 108 by increasing the amount of mass the second motor 108 is required to move. The stiffness of the drive mechanism is also a function of the mass of the system. The higher the mass, the stiffer the drive mechanism needs to be for a desired movement in a desired time.
Another problem of having the first motor 116 mounted on the movable X stage 104 is the requirement to route power to the motor. Because the motor 116 moves back and forth as the X stage 104 moves, the power connection must be flexibly mounted from base 102 up to the X stage 104. This flexible power connection introduces wear problems that are costly to overcome.
Another problem of having the first motor 116 mounted on the movable X stage is the magnetic field (not shown) generated by the motor 116 moves as the motor 116 moves. This moving magnetic field creates problems for the X-ray beam (not shown) by changing the direction of the X-ray beam dependent on the location of the magnetic field radiating from the motor 116. This problem requires shielding to prevent the X-ray beam from moving or some other means to compensate for the movement of the X-ray beam.
Another problem with having a motor mounted on a moving stage is the difficulty in isolating the moving motor from the environment that may be required by the work piece. For example in some integrated circuit manufacturing steps the workpiece (the silicon wafer) must be kept extremely clean. Maintaining this type of environment with a motor moving through the environment is difficult.